Apparatus for balancing helicopter rotors typically require a "one-per-rev" signal for timing and other purposes. Two methods are currently popular for obtaining such signals. One method is to connect to the tachometer-generator which is mechanically connected to the main rotor. The output of this device is typically two sine waves 90.degree. out of phase with each other. This signal provides angular velocity, phase and directional information for the main rotor. The drawback to using this signal is that it requires tapping into the helicopter's wiring harness and conditioning the signal for the various types of tachometer-generators.
The second method of obtaining a tachometer signal is to mount a coil wrapped around a magnetic core near the main rotor shaft. A ferromagnetic plate is attached to the main rotor shaft and passes in close proximity (approximately 1/4 inch) to the coil as the shaft rotates. The reluctance of the air gap between the plate and the coil changes and induces a voltage across the coil when the plate passes. Such devices are known as magnetic interrupters, and brackets to mount them are found on most helicopters. The major disadvantage to using magnetic interrupters is the close mechanical tolerance for alignment of the plate and coil. If they are too distant, the interrupter output is degraded, and if they are too close, they may physically contact one another resulting in mechanical failure and potential harm to the helicopter power train from foreign object damage (FOD). An additional disadvantage to using magnetic interrupters is the weight of the interrupter plate, which can cause significant imbalance in small rotor shafts. For these reasons, magnetic interrupters have begun to be replaced by optical units.
The earliest optical units utilized a beam of visible or infrared light generated by a lamp to illuminate a reflector on the rotor shaft. A receiver detected the reflected beam. The added range provided by the optical unit decreased the need for close mechanical tolerances required by magnetic interrupters. These early optical devices were sensitive to the shadows of the helicopter blades, changes in ambient light, strobe light interference and several other sources of optical noise, however. To solve this problem, most optical units now modulate the emitted beam and trigger only when they detect a beam with the correct carrier frequency. As a result of the necessity to modulate the beam and the modulation frequencies required, virtually all optical units now use a light emitting diode (LED), an infrared emitting diode (IRED), or a laser diode to generate light. Because the highest power output diode devices emit in the infrared region, and because silicon detectors frequently enjoy highest sensitivity in the infrared region, virtually all devices on the market for helicopter optical tachometer sensors utilize an infrared beam.
Infrared beams are invisible to the human eye. Infrared optical sensors for helicopters are thus difficult to align. Devices which emit visible light are also difficult to align during the daytime because high ambient light often washes out beam strength so the user cannot tell where the beam hits the helicopter rotor shaft. There is thus a need for a device which can be boresighted by the user despite the fact that it uses invisible light, or, if it uses visible light, can be boresighted during daytime.
Another drawback to current optical devices is dictated by their predecessors, the magnetic interrupters. Since so many helicopters are already outfitted with mounting brackets for the magnetic interrupters, there is very strong motivation to make an optical unit compatible with the magnetic interrupter footprint. The magnetic units were very small, however, leaving very little room for the added electronics required by the optical unit. As a result, many (if not all) manufacturers have an optical sensing head which approximates the size of the magnetic interrupter unit, and a signal conditioning unit which typically must be mounted within close proximity (less than five feet) to the optical sensing head. This additional unit must be secured to the exterior of the aircraft and withstand abuse caused by weather, airspeeds approaching 200 miles per hour and other environmental conditions resulting from the external location.